Notch filters are often used to reduce acoustic feedback in sound amplification systems, including public address systems and music delivery systems in which microphones and speakers are deployed. For example, U.S. Pat. No. 4,091,236 (Chen, issued May 23, 1978) describes an analog notch filter for an audio signal to suppress acoustic feedback. The apparatus receives an audio signal that is substantially non-periodic in the absence of acoustic feedback and substantially periodic with an instantaneous dominant frequency in the presence of such feedback.
The duration of successive periods are monitored and compared by an up/down counter to determine whether the audio input signal is substantially periodic and to determine the instantaneous dominant frequency of the audio signal. Upon detection of an audio signal that is substantially periodic, the notch filter is tuned to the instantaneous dominant frequency so as to suppress the acoustic feedback.
U.S. Pat. No. 4,232,192 (Beex, issued Nov. 4, 1980) describes an integrator/detector that determines when an audio signal has exceeded a threshold for a selected number of cycles. If the threshold is exceeded for the selected number of cycles, a sampler circuit samples a voltage corresponding to the frequency that has exceeded the threshold. The sampled voltage is used by a voltage frequency converter in order to adjust the notch of a notch filter implemented in hardware.
U.S. Pat. No. 5,245,665 (Lewis et al., issued Sep. 14, 1993) describes a device for suppressing feedback in which a Fast Fourier Transform is conducted on samples of digitized signals to produce corresponding frequency spectrums. The magnitudes of the spectrum at various frequencies are analyzed to determine one or more peak frequencies that are 33 decibels greater than harmonics or sub-harmonics of the frequency in an attempt to detect resonating feedback frequencies. Two processors are required. A primary processor periodically collects a series of the passing digital signals and conducts a Fast Fourier Transform on each collected series of digital signals. The frequency spectrums produced by the Fast Fourier Transform are examined by the primary processor to discover the presence of any resonating feedback frequency. Filter control signals are passed by the primary processor, along with the digital sound signals, to a secondary processor which operates a digital filtering algorithm in accordance with the filter control signals to attenuate resonating feedback frequencies in the stream of digital signals.
U.S. Pat. No. 5,999,631 (Porayath, et al., issued Dec. 7, 1999) employs an algorithm defining a digital filter with a notch adjustable to a plurality of notch values for a single acoustic channel. Feedback is located by comparing values resulting from the processing with the notch adjusted to different notch values. Notch filter coefficients are generated directly by the feedback detector, in which feedback is detected if a first harmonic component is sufficiently small relative to a fundamental component. The notch values are adjusted until the signals processed by the notch filter algorithm result in a minimum mean squared value over a time window. After the feedback has been located using the notch filter algorithm, digital output signals are generated by executing the algorithm with the notch adjusted to the notch value at which the minimum mean squared value results. The digital output signals then are converted to corresponding analog signals that are transmitted to a speaker.
The approaches of the prior art, as described heretofore, reduce acoustic feedback on an individual channel basis. However, public address systems and music delivery systems may utilize a plurality of channels, such as a stereophonic (2-channel) acoustic system. A listener is typically very sensitive to any differences of operation between channels (often referred as “stereo image”), even though the differences may seem insignificant in an absolute sense. Furthermore, a notch filter may be deployed on one channel and not the other. Such an occurrence would cause a higher degradation of the stereo image. Thus, it would be an advancement of the art to provide apparatuses and methods that assist in balancing a plurality of acoustic channels for an acoustic delivery system.